Luminescence refers to light emission caused by processes excluding incandescence, and including processes such as fluorescence, phosphorescence, chemiluminescence, bioluminescence, and electroluminescence. Luminescent materials are useful for sensors through modification of either transient or steady-state emission by an analyte. Scintillators, also referred to herein as “scintillating materials,” comprise one class of luminescent sensor material, and generally refer to materials which emit prompt luminescence when exposed to ionizing radiation. When excited by ionizing radiation, electrons may be freed from atoms of the luminescent material. The electrons and molecular ions recombine to form neutral states, so-called ‘singlet’ and ‘triplet’ excitation states. Singlet excited states generally refer to states in which the excited electron is paired with a ground state electron, in that the excited and the ground state electron have an opposite spin. Triplet excited states generally refer to states in which the excited electron is not required to be paired with a ground state electron. Approximately 25% of the electrons excited by ionizing radiation in a scintillating material may relax to singlet excited states, while 75% of the excited electrons may go to a triplet state.
As the excited electrons relax to a ground state, they may emit luminescence. FIG. 1 is a schematic illustration of transitions experienced by excited electrons, as known in the art. Excited electrons occupying singlet states (S1 102) may generally freely relax to a ground state (S0 103), emitting luminescence 110. The luminescence 110 generated based on relaxing singlet states is generally considered ‘fast’ luminescence, occurring on the order of nanoseconds, and is based on the singlet state excited electron making a direct radiative transition to the ground state in the scintillating material. Excited electrons in triplet states (T1), such as T1 112 and 114 however, may not freely relax to a ground state. A transition from a triplet state to a ground state is ‘spin-forbidden’ according to the laws of physics. Instead, pairs of excited electrons in triplet states 112 and 114 may recombine with one another to generate an excited electron in a singlet state 116 and another 118 in a ground state. The excited electron in the singlet state 116 may then relax to a ground state S0 120 and generate luminescence 122. The triplet states accordingly often produce little or no luminescence, due to nonradiative transitions, or a slower luminescence, on the order of milliseconds or longer.
In some organic scintillating materials delayed singlet luminescence may be observed. The rate of this delayed luminescence is determined by the rate of diffusion of the triplet states to combining with one another within the scintillating material. Accordingly, the delayed luminescence may experience a non-exponential decay. Typically, only a small fraction (such as two percent) of excited electrons in triplet states may undergo this recombination and relaxation to produce luminescence.
The ‘fast’ and ‘delayed’ luminescence components described above may be exploited in scintillator systems to discriminate between exciting particles. For example, scintillating materials may be used to discriminate between energetic neutrons and gamma ray photons. These neutral particles must be converted to charged particles by the sensor material in order to be detected. Neutrons are observed based on their creation of recoil protons in the scintillating material, whereas gammas are converted to fast electrons, as understood in the art. Particle discrimination is possible in part because the ‘fast’ luminescence described above is generally dependent on the energy deposited per unit distance in the scintillating material, which is lesser for electrons than protons. FIG. 2 is a schematic illustration of the intensity of luminescence generated by ionizing electrons and recoil protons, respectively, over time. Luminescence generated by an electron is illustrated by line 202. Luminescence generated by a recoil proton is illustrated by line 204. As illustrated, an initial ‘fast’ luminescence intensity 210 may vary according to dE/dx, and therefore differ between the electron and recoil proton, with the electron producing a greater fast luminescence. However, the ‘delayed’ luminescence 212 generally does not vary by particle type. This effect may be used to differentiate signals from the different particle types, a technique referred to as pulse shape discrimination (PSD).
FIG. 3 is a schematic illustration of a scintillator system 310 configured to perform PSD. A photomultiplier tube (PMT) 304 is positioned to receive luminescence generated by a scintillating material 302. PMT 304 generally may convert the luminescence to an electronic signal. Electronics 306 may be coupled to the PMT 304 and receive the electronic signal generated by the PMT 304. The electronics 306 may be configured to detect particles and/or discriminate between particle types based on the temporal signature of the luminescence generated by the scintillating material 302.
Recall, as described above, generally few excited electrons in triplet states in the scintillating material relax to generate luminescence. Rather, a greater number of singlet state electrons relax and generate luminescence. The heavy-atom effect has been used to increase the availability of transfer states for excited triplet states. Briefly, the heavy atom effect refers to an effect whereby the presence of a heavy atom accelerates the transition of excited triplet states to a ground state. The heavy-atom effect has been used, for example, to identify the triplet absorption state in stilbene. See Dyck, R. H. et al., “Ultraviolet spectra of stilbene, p-monohalogen stilbenes, and azobenzene and the trans to cis photoisomerization process,” Journal of Chemical Physics, 36, p. 2326 (1962), which article is hereby incorporated by reference in its entirety for any purpose. Further, the heavy atom effect has been used to create fast-emitting phosphors for organic light-emitting diodes. See Thompson, M. “The evolution of organometallic complexes in organic light-emitting diodes,” MRS Bulletin, 32, p. 694 (2007), which article is hereby incorporated by reference in its entirety for any purpose. Further, the heavy-atom effect has also been shown to increase light yield in plastic scintillators. See I. H. Campbell, et. al., “Efficient plastic scintillators using phosphorescent dopants,” Applied Physics Letters, 90, p. 012117 (2007), which article is hereby incorporated by reference in its entirety for any purpose.